Peel the Vidalia or sweet onion and slice it into rings about 1/2 inch thick. Separate the rings and set them aside.
In a mixing bowl, combine the flour, cornstarch, paprika, garlic powder, salt, and black pepper. Mix well.
In another bowl, mix the plant-based milk and apple cider vinegar (or lemon juice). Let it sit for a few minutes to curdle slightly.
Pour the milk mixture into the dry ingredients and stir until you have a smooth batter. The batter should be thick enough to coat the back of a spoon.
Heat vegetable oil in a deep skillet or pot to around 350°F (175°C).
Dip each onion ring into the batter, letting any excess drip off. Coat the ring evenly and let any excess batter drip back into the bowl.
Carefully place the battered onion rings into the hot oil, a few at a time, to avoid overcrowding the pot. Fry them for about 2-3 minutes on each side, or until they are golden brown and crispy.
Use a slotted spoon to remove the onion rings from the oil and place them on a plate lined with paper towels to absorb any excess oil. Sprinkle with a little extra salt while they're still hot.
Serve the vegan onion rings while they're still warm and crispy. They're great on their own or with your favorite dipping sauce.